This invention relates generally to microwave apparatus for safely heating a batch of distinct articles and, more specifically, to such an apparatus for thawing the frozen parenteral solution content of plastic bags.
The addition of medication to parenteral solutions is a widely accepted clinical practice. Although many intravenous additives presently are prepared in patient care areas by nursing staff, there is an increasing pressure on hospitals to establish pharmacy admixture programs to minimize medication errors and the risk of microbial contamination. To optimize the use of available manpower, some pharmacies have adopted the practice of freezing admixtures.
It is known that several antibiotics in clinical use are stable for extended periods of time when frozen in large volume parenteral solutions. Indeed, freezing of antibiotic solutions often can prolong stability and suspend growth of essentially any microbial contaminant. Studies have shown that antibiotic admixtures are unharmed for up to 30 days if frozen in 100 or 50 ml. admixture container bags. However, the several hours required for room temperature thawing of these units often make this technique unsuitable in the hospital. Some improvement in preparation efficiency can be achieved by thawing the bags in warm water baths, a technique that is widely practiced in blood banks. Warm water thawing, however, still consumes significant time in addition to requiring labor intensive procedures for both cleaning baths to reduce contamination levels and avoiding contamination by touch during pooling operations.
As a solution to these problems, microwave ovens have been proposed as vehicles for thawing frozen admixtures prior to intravenous administration. The resultant rapid thawing would allow centralized admixture programs to operate a 24-hour service without the need for a pharmacy to be ready to make admixtures at all times. Other attributes would include the opportunity to take advantage of the efficiency accompanying large scale manufacturing techniques. Especially important would be the possibility of introducing pre-release quality control on manufactured lots, presently not possible for admixtures prepared by conventional methods where the maximum permissible refrigerated storage is 24 hours.
Although the efficiency of microwave heating presents many advantages, prior microwave ovens do not satisfy all requirements for safely thawing individually bagged units of parenteral substances on a scale necessary for practical applications. Even the most sophisticated timing and temperature sensing controls available with existing microwave ovens cannot prevent under all conditions the overheating of individual units during batch thawing thereof. For example, the inadvertent combination, in a given batch, of units exhibiting substantially different volumes or chemical characteristics can cause non-uniform temperature increases and resultant overheating of specific units. Similar consequences can derive from inadvertently including a thawed or partially thawed bag with a fully frozen batch. In addition to possibly degrading the therapeutic effectiveness of its content, overheating can cause rupture of an individual bag. Under certain conditions the explosive discharge from a ruptured bag can injure attendant personnel and in all cases must be cleaned from the equipment surface on which it collects.
The object of this invention, therefore, is to provide microwave oven apparatus that can safely, efficiently and simultaneously thaw the frozen parenteral mixture content of multiple plastic bags.